Electric discharge devices



Juh 10, 1958 M c. CROWLEY-MILLING ELECTRIC DISCHARGE DEVICES Filed July 18, 1952 \NVENTOR ATTORNEY5 United States Patent Office ELECTRIC DISCHARGE DEVICES Michael Crowley Crowley-Milling, Colwyn Bay, Wales, assignor to Metropolitanickers Electrical Company Limited, London, England, a company of Great Britain Application July 18, 1952, Serial No. 299,616

Claims priority, application Great Britain July 27, 19 51 2 Claims. (Cl. 315-39) This invention relates to electric discharge devices of the kind in which a beam of charged particles is projected along a path within an evacuated container.

The invention has an important application inter alia in linear accelerators employing electron beams. In such apparatus the beam is normally projected along a path Withinan evacuated container between an electron gun and a target. In order to prevent beam-spread laterally it is necessary top-rovide some form of focusing for the beam. A common form consists of an electric coil distributed along the outside of the container. Such a coil is usually of a large number of turns with a relatively small exciting current.- Such an arrangement may be made to operate satisfactorily, but it requires a large amount of copper in relation to the magnetic field produced along the axis. Furthermore, for purposes o evacuating the container of the device it is necessary to provide an exhaust pipe to the pumping apparatus and this exhaust pipe usually must pass between the turns of the focusing winding, and the necessary gap in the coil to permit passage of the exhaust pipe will cause distortion of the focusing field. 7

According to the present invention the focusing winding is located within the container of the apparatus so as to surround the path of the beam, the ends of the winding passing through the container wall with suitable insulation, the winding being formed of a coil of comparatively few turns of tubing and fluid-cooled, excited with a relatively heavy current.

It will be appreciated that with such an arrangement the turns of the tubing will be spaced sutficiently to alloW evacuation of the central volume without having to modify the shape of the windings from that most desirable from magnetic field considerations. In addition, closer control can be exercised over the axial distribution of the magnetic field due to the smaller diameter of the coil. Furthermore, there is a reduction in the weight of copper required since the fluid cooling of the tube permits a higher current density and also on account of the reduction in diameter of the focusing coil resulting from locating it inside the container.

In order that the invention may be clearly understood reference will now be made to the accompanying drawing, which shows an elevation, partly in section, of a linear accelerator embodying the invention.

In the apparatus shown the beam is projected downwardly from an electron gun located in the part 1 along the waveguide tube 2 having peripheral apertures 30 to a target located at 3, and X-rays are emitted downwardly through a collimator 4. The apparatus is mounted in an evacuate-d container 5.

The references 6, 7, and 8 indicate parts of a waveguide system by which travelling Waves are projected downwardly along the accelerator tube 2. In actual practice the tubes 7 and 8 will be interconnected, though for convenience these are shown broken.

The focusing magnetic winding 9 for the purpose of focusing the beam extends along the tube 2 so as to act 2,838,711 Patented June 10, 1958 upon the electron beam throughout the greater part of its travel in the container 5, which is evacuated through the tube 29.

In arrangements according to the invention the focusing coil is formed of tubing through which a cooling fiuid, which is preferably a liquid such as oil or water, is passed. There are a relatively small number of turns of the tubing as compared with the conventional form of focusing winding, and this is possible since a much higher current density is permissible on account of the fact that the tubing is fluid-cooled. Since relatively few turns of tubing are used these can bespaced apart by rigid spacers so that the coil may be placed immediately surrounding the accelerator tube, inside the vacuum envelope, tliere being thus a free path. between the turns for the evacuation of the accelerator tube through theapertures 30. Moreover, by mounting the tubing within the evacuated container instead of outside it the diameter is appreciably reduced with a consequent considerable saving in copper and the smaller diameter of the focusing coil enables a closer control to be exercised over the axial distribution of the field. The ends of the tubing are connected to lead-in sections 10 and 11 passing through insulating bushings 12 and 13 respectively secured to the walling of the container 5. As will be appreciated, the ends 10 and 11 are connected both to'the cooling fluid supply and also to the electrical supply; The bushings 12 and 13 may, for example, be steatite or ceramic bushes sealed both to the container wallingand to the lead-in tubing. In the actual arrangement shown the winding is carried on spacers supported by a tubular insulating former 14 surrounding the accelerator tube 2. Former 14 has perforations 28 to allow waveguide tube 2 to be evacuated therethro-ugh. In some cases, of course, the insulating former may be dispensed with and the tubing supported by the leads-in.

In some cases a uniform focusing field may be required along the length of the accelerator, but in other cases a uniform field may not be required, and in the arrangement shown the number of turns per unit is progressively reduced so that the focusing field is progressively reduced along the length of the accelerator.

In cases in which a locally increased field is required the turns of the winding may be spaced more closely, and the number of layers varied, and in certain cases this field may be still further enhanced by permanent magnets, e. g. of Alnico, located inside the container, and extending axially. A particular case may arise where it is desired to increase the focusing field in the neighbourhood of the gun.

One such arrangement in the example shown consists of a transverse end plate of magnetic material 15 and axially extending permanent magnets all of the same polarity 16 so that the axial field is enhanced through the interior of the winding.

Alternatively an annular magnet magnetised axially might be employed.

Local reduction of the focusing field may be effected by connecting shunts across certain turns.

The electrical supply to the winding may be from a suitable heavy current source 17 over leads 18, 19. The cooling fluid in the coil 9 is circulated by a pump indicated schematically by the rectangle 20 and passes through suitable cooling apparatus indicated schematically by the rectangle 21. Supply pipes 22, 23 connect the pump 20 and cooling apparatus 21 with the tubing 9, the pipes passing through the bushings 12 and 13 to which the leads 18, 19 are secured by means of suitable lugs. The interior of the waveguide tube 2 is corrugated by apertured diaphragms 26 suitably spaced along it so as to accelerate the electrons in a manner which is well known.

The focusing coil is supported by three insulating spacers located 120 apart around the perimeter of the guide and parallel to its axis. Two such spacers can be seen in the drawing. Holes are drilled in these spacers for the turns of the focussing coil to be supported. The spacers are preferably made of Mycalex.

What I claim is:

1. In a linear accelerator for an electron beam, a waveguide having peripheral apertures therein, an electron gun for projecting a beam of electrons into said waveguide, apertured diaphragms spaced along said wave-guide for accelerating the electrons to strike a target, a container surrounding said waveguide and communicating therewith, means for continuously evacuating said container and the waveguide through its peripheral aperture, a coil of hollow tubing located within said container and encircling the waveguide, an insulating former for said coil located within the container around the outside of the waveguide, means for separating the turns of the coil with spacing which provides free paths for the evacuation of the waveguide through the apertures therein, insulating and vacuum tight bushings in the container through which said tubing is passed, means for circulating a unidirectional electric current through said tubing thereby to create a magnetic focussing flux in said waveguide, and means for circulating cooling liquid through the interior of said tubing from outside the container.

2. In a linear accelerator for an electron beam, a waveguide having peripheral apertures therein, an electron gun for projecting a beam of electrons into said waveguide, apertured diaphragms spaced along said waveguide for accelerating the electrons to strike a target, a container surrounding the waveguide and communicating therewith, means for continuously evacuating the container and the Waveguide through its peripheral apertures, a, coil of hol low tubing located within said container and encircling the waveguide, an insulating former for said coil located Within the container around the outside of the Waveguide, means for separating the turns of the coil with spacing which varies along the waveguide and provides free paths for the evacuation of the waveguide through the apertures therein, insulating and vacuum tight bushings in the container through which said tubing is passed, means for circulating a unidirectional electric current through said tubing thereby to create a magnetic focussing flux in said waveguide, means for circulating cooling liquid through the interior of said tubing from outside the container, and permanent magnet means for producing an auxiliary axial magnetic field adjacent the electron gun.

References Cited in the file of this patent UNITED STATES PATENTS 1,328,336 Northrup Jan. 20, 1920 1,562,172 Houskeeper Nov. 17, 1925 1,753,408 Gebhard Apr. 8, 1930 1,985,324 McCullough Dec. 25, 1934 2,234,281 Ruska Mar. 11, 1941 2,244,752 Wolif June 10, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,442,975 Grundmann June 8, 1948 2,444,368 Reisner et al June 29, 1948 2,521,556 Wilbur Sept. 5, 1950 2,541,843 Tiley Feb. 13, 1951 

